Thursday, May 19, 2016

Bright, frosty polar caps, and clouds above a vivid, rust-colored
landscape reveal Mars as a dynamic seasonal planet in this NASA Hubble
Space Telescope view taken on May 12, 2016, when Mars was 50 million
miles from Earth. The Hubble image reveals details as small as 20 to 30
miles across.

The large, dark region at far right is Syrtis Major Planitia, one of
the first features identified on the surface of the planet by
seventeenth century observers. Christiaan Huygens used this feature to
measure the rotation rate of Mars. (A Martian day is about 24 hours and
37 minutes.) Today we know that Syrtis Major is an ancient, inactive
shield volcano. Late-afternoon clouds surround its summit in this view.

A large oval feature to the south of Syrtis Major is the bright
Hellas Planitia basin. About 1,100 miles across and nearly five miles
deep, it was formed about 3.5 billion years ago by an asteroid impact.

The orange area in the center of the image is Arabia Terra, a vast
upland region in northern Mars that covers about 2,800 miles. The
landscape is densely cratered and heavily eroded, indicating that it
could be among the oldest terrains on the planet. Dried river canyons
(too small to be seen here) wind through the region and empty into the
large northern lowlands.

South of Arabia Terra, running east to west along the equator, are
the long dark features known as Sinus Sabaeus (to the east) and Sinus
Meridiani (to the west). These darker regions are covered by dark
bedrock and fine-grained sand deposits ground down from ancient lava
flows and other volcanic features. These sand grains are coarser and
less reflective than the fine dust that gives the brighter regions of
Mars their ruddy appearance. Early Mars watchers first mapped these
regions.

An extended blanket of clouds can be seen over the southern polar
cap. The icy northern polar cap has receded to a comparatively small
size because it is now late summer in the northern hemisphere. Hubble
photographed a wispy, afternoon, lateral cloud extending for at least
1,000 miles at mid-northern latitudes. Early morning clouds and haze
extend along the western limb.

This hemisphere of Mars contains landing sites for several NASA Mars
surface robotic missions, including Viking 1 (1976), Mars Pathfinder
(1997), and the still-operating Opportunity Mars rover. The landing
sites of the Spirit and Curiosity Mars rovers are on the other side of
the planet.

This observation was made just a few days before Mars opposition on
May 22, when the sun and Mars will be on exact opposite sides of Earth,
and when Mars will be at a distance of 47.4 million miles from Earth.
On May 30, Mars will be the closest it has been to Earth in 11 years,
at a distance of 46.8 million miles. Mars is especially photogenic
during opposition because it can be seen fully illuminated by the sun as
viewed from Earth.

The biennial close approaches between Mars and Earth are not all the
same. Mars' orbit around the sun is markedly elliptical; the close
approaches to Earth can range from 35 million miles to 63 million
miles.

They occur because about every two years Earth's orbit catches up to
Mars' orbit, aligning the sun, Earth, and Mars in a straight line, so
that Mars and the sun are on "opposing" sides of Earth. This phenomenon
is a result of the difference in orbital periods between Earth's orbit
and Mars' orbit. While Earth takes the familiar 365 days to travel
once around the sun, Mars takes 687 Earth days to make its trip around
our star. As a result, Earth makes almost two full orbits in the time it
takes Mars to make just one, resulting in the occurrence of Martian
oppositions about every 26 months.